Taste buds and olfactory epithelium each consist of several morphologically-identifiable cell types. The proposed experiments will examine whether these different cell types arise from different progenitor cells and, for taste buds, whether different morphological types are related to the immunochemically distinct cells in a taste bud. In order to examine lineage relationships within chemosensory epithelia, chimeric mice will be prepared in which embryos from two different mouse strains are fused early in development. The resulting chimeric mouse consists of tissues exhibiting a mosaic of cells derived from one or the other strain. The pattern of mosaicism in relationship to the different morphological types of cells identifiable in the chemosensory epithelia will permit determination of whether the different cell types originate from a common precursor or from different precursors. For example, the patterns in the chimeric mice will permit determination as to whether dark and light cells within a taste bud arise from different progenitors or whether these taste cells represent endpoints of a single lineage. Further, these experiments may reveal whether taste buds are clonal populations of receptor cells derived from single progenitors. If so, the number of such taste bud progenitor cells can be determined. Similarly, chimeric mice will be used to study the origins of the various cell types in the olfactory epithelium: receptors, supporting cells and microvillar cells. Whether these three cell types originate from a common precursor should be determinable. Other studies in this proposal will compare cytochemically different cells within a taste bud to determine whether the cytochemical differences reported are related to cell morphology or to functional status. The focus of these investigations will be taste cells expressing robust NCAM immunoreactivity. Such cells do not fit well into a single morphological type (dark, intermediate, light) and the presence of NCAM may reflect the synaptic status of the cell. Reconstructions of serial sections taken through the NCAM-immunoreactive cells will determine whether these cells contact nerve fibers, and if so, whether they form synaptic contacts.